1. Field of the Invention
The present invention relates to ceramic heaters and gas sensor elements using the ceramic heater. In particular, the ceramic heater has a structure in which a conductive layer is formed on a plate shaped base member made of ceramic.
2. Description of the Related Art
Ceramic heaters are widely used to various applications which are required to generate heat in order to quickly increase a temperature of a target section. For example, a ceramic heater is used to a gas sensor to be mounted on an exhaust gas pipe connected to an internal combustion engine. When receiving electric power, the ceramic heater generates and supplies heat to a solid electrolyte body in a gas sensor in order to speedily increase a temperature of the solid electrolyte body and a pair of electrodes formed on the solid electrolyte body so that the temperature of the solid electrolyte and the electrodes quickly reach an activation temperature thereof.
For example, a patent literature 1, Japanese patent laid open publication number S62-167396, shows a ceramic heater having a heating resistance pattern formed on a heat resistance ceramic substrate so that a central section and both end sections of the heating resistance pattern have a different line width or a different line thickness in order to approximately have a uniform temperature distribution thereof. In particular, the heating resistance pattern has a structure in which the linear shaped central section has the line width which is larger than a line width of the both end sections. Booth end sections become a low heating temperature which is lower than that of the central section in the heating resistance pattern.
By the way, because the recent vehicle emissions control of reducing motor vehicle emissions, etc. is becoming stricter year by year in view of environmental protection, there is a demand to turn off electric power to be supplied to in-vehicle electronic devices such as a gas sensor during an idling stop control. However, when a ceramic heater in a gas sensor is repeatedly turning on and off, the ceramic heater is repeatedly expanded and contracted. This deteriorates the heating section in the ceramic heater and there is a possible reduction of a product life of the ceramic heater.